Canon has holes in its lineup, including this one. Canon will fill it if it finds it profitable to do so after the higher priority lenses are released.
That would require Canon to eat humble pie and compete directly with existing third-party options, something they've never been willing to do.
They'd rather ignore the massive dominance of 150-600 lenses and release a 100-500 just so they don't seem to be copying Tamron. It's all about self-image.